Activated carbon is used to remove any number of materials from liquids and gasses, for recovery of values or for the purification of a wide range of substances. It is used in the water, food, mining, automotive, chemicals, pharmaceutical and environmental industries. A common application of activated carbon is to adsorb ions, complexes and molecules from aqueous solutions, and the like. Accordingly, activated carbon has been used to extract dissolved metals, either to purify the water, and/or to recover valuable metallic values.
In particular, in water treatment applications, activated carbon is commonly used for removing organic molecules and heavy metals. Water is commonly adulterated with pollutants from industrial activity, runoff from polluted sites, and from various other sources. These pollutants, which can range from various organic molecules and heavy metals must be removed to render the water safe and to comply with environmental regulations. This is often accomplished by contacting the polluted water with activated carbon. The treated water and the activated carbon are then separated, and the activated carbon is treated to dispose of the pollutants. The water is often contacted with the carbon by passing the water through fixed beds of the carbon. However, some operational efficiencies may be derived by mixing the water and carbon in, for example, a stirred tank. However, this requires separation by mechanical screening, or the like, to separate the carbon particles from the treated water.
Another common application of activated carbon is to extract gold complexes from gold leach solutions. Gold leaching in alkaline cyanide solutions has been studied extensively for more than 200 years, and it has been applied successfully at the industrial level for more than a century. The high recoveries, economics and simplicity of the process have made cyanide leaching the preferred route for gold dissolution from its ores. Conventional gold recovery methods involve crushing and grinding of the gold ores followed by dissolution of the gold in an oxygenated alkaline cyanide solution. Gold, dissolved as the Au(CN)2− complex, is recovered from solution by several methods, adsorption on activated carbons (carbon-in-pulp, carbon-in-leach and carbon-in-column processes) being a preferred method. The United States mined 340 metric tons of gold during 1998, of which 78% were recovered by the use of activated carbon. The remaining 22% was recovered by other methods. After separation of the activated carbon from the pulp or solution, the adsorbed Au(CN)2− complex is stripped off the activated carbon for subsequent precipitation and refining.
Activated carbons are used for gold recovery from solutions due to their high capacity for adsorption. The adsorptive properties of activated carbon are a consequence of the highly developed micropore structure and of the surface functional groups generated during the production process. Gold adsorption onto activated carbon is a diffusion-controlled process, where the size of the carbon particles plays an important role. In general, gold adsorption capacity and gold adsorption kinetics increase as the size of the activated carbon particles decrease. In spite of the advantages that fine activated carbon particles offers in terms of adsorption capacity and adsorption kinetics, conventional carbon-recovery gold circuits use granular activated carbon particles that are significantly coarser than the ground ore, in order to produce an effective separation of the gold-loaded activated carbon from the slurry phase by mechanical screening. Mechanical screening is the simplest method of separation of solids, and it is based solely on the difference in particle sizes.
Separation of activated carbon from the slurry phase by screening has significant problems. The screens may blind and break due to excessive wear, require considerable horsepower to operate, and need frequent maintenance and screen replacement. Also, gold loaded onto the exposed carbon may be lost by abrasion. Losses of activated carbon due to these factors could be as high as 50 grams of carbon per metric ton of ore, and cumulative losses of fine activated carbon within a year of operation can be as high as 20%. This represents significant amounts of gold adsorbed to activated carbon that is unrecoverable by screening. Fine powdered activated carbon shows a significant increase in gold adsorption kinetics and gold adsorption capacity due to a more extended external surface area. However, powdered activated carbon currently cannot be used in gold recovery circuits, since both the carbon and the solids in suspension will have particle sizes of the same magnitude, thus, separation of the gold-loaded activated carbon from the slurry phase by screening, filtration, or sedimentation are not viable options.
In general, a method to separate activated carbon particles from a liquid solution that avoids the problems of mechanical screening would be desirable. Since screening depends upon the particles having a relatively large size, abrasion or other unintended comminution, produces small particles which defeats the screening. An alternative to screening would be magnetic separation, using a magnetic activated carbon. Magnetic carbons have been made by mixing or coating carbon or a carbon precursor with a magnetic material, usually with magnetite, and treating to activate or transform the carbon or carbon precursor. A problem with these materials is that the magnetic material is widely dispersed in the carbon particle or upon its surface. This is inherent in these compositions, since powdered magnetic material (magnetite) is used and in only a minor amount to impart the magnetic property. Thus, depending on the particular composition, the magnetic material is dispersed throughout a matrix as small particles of the carbon material, or concentrated upon the surface. When these materials are abraded or comminuted, carbon fines are usually formed that are free the magnetic material and cannot be magnetically separated. Thus, magnetic separation requires that the particles be relatively large to maintain their magnetic properties. Accordingly, the non-magnetic fines formed cannot be recovered. Thus, the recovery requires relatively large carbon particles, the same as in mechanical screening, and fine carbon particles cannot be recovered.
Additionally, the abrasion of the prior-art magnetic activated carbon materials also frees the magnetic material. This magnetic material must then be removed by screening. However, unfortunately, this screening also separates the abraded non-magnetic carbon fines with the freed magnetic particles, both of which are then lost.
Another problem with some magnetic carbon compositions, is that their magnetic properties are not sufficiently “soft.” It is desired that the carbon become magnetic only during exposure to the magnetic field used to separate the carbon particles from the solution. Any residual or “hard” magnetism that remains only complicates the separation process since the carbon particles stick together and stick to and foul the process machinery.
There is a need in the art for a magnetic activated carbon that has soft magnetic properties, where the magnetic property is essentially homogeneously dispersed or intimately mixed throughout the activated carbon so that even small carbon fines retain the magnetic property. This would allow recovery of the carbon, independent of the particle size and totally eliminate the need for screening. Smaller activated carbon particles (even powdered) could then be used to exploit kinetic and adsorption capacity advantages of small particles.